Currently, information transmission, for example data and video transmission for cable television (CATV) and wireless communication systems, often utilizes long-haul fiber-optic links. In order to take advantage of the enormous bandwidth that the optical fiber provides, the optical carrier has to be modulated and transmitted at high rates. Directly modulated laser diodes have been used for this purpose, however, these modulated laser diodes are known to be susceptible to frequency chirping. This frequency chirping problem can be avoided by employing external modulators. Accordingly, analog systems based on efficient external modulators are highly desirable for many commercial and military applications, for example sensor systems, fiber-optic telecommunication links, and microwave antenna remote detection systems.
Guided-wave electro-optic modulators in which light is confined within a waveguiding area of small transversal size are promising candidates for such applications. Although several commercial integrated-optic (IO) modulators are currently available, distortions caused by nonlinearities in their modulation curves severely degrade their performance. Typically, these modulation curves exhibit sine-squared behavior, thus hindering widespread deployment of integrated-optic modulators in high-performance analog optical systems typically requiring that nonlinear distortions be 95 dB below the carrier.
A number of linearization techniques have been developed to suppress the nonlinear distortions produced by 10 modulators. In general, these techniques fall into two categories, namely, electronic compensation and optical methods of linearization. Electronic techniques, based on predistortion compensation or feedforward compensation, involve expensive high-speed electronic components and are limited by a bandwidth of a few GHz or less. Optical techniques include the dual-polarization technique, the use of two- and three-section directional couplers, parallel modulation schemes, and various cascaded schemes. Common to all these techniques is that the improvement in linearity is achieved at the expense of more complex device designs, particularly in the case of cascaded schemes involving multiple modulator structures. Many of these techniques employ multiple electrode sections and, as a result, require several separate modulating sources and/or several bias controls. The need for several modulating sources and bias controls seriously hampers the use of these linearized devices in high-speed applications where efficient and convenient matching of the electrode structure to the microwave source is required. In addition, complex devices are prone to thermal and temporal instabilities that make these schemes unattractive for practical applications.
Accordingly, there is a need for an integrated-optic modulator with highly linear performance, simple design, and reduced sensitivity to fabrication deviations. In addition, a modulator without high-speed electronic components for linearization would be advantageous. Furthermore, a modulator with a simple uniform electrode structure which can be conveniently matched to a single microwave driving source would be particularly advantageous. Also, a modulator which can be utilized at high speeds would be particularly advantageous.